Composite fiber tape formed of high tensile strength fibers of materials such as boron, graphite, or glass, and impregnated with a matrix of a thermosetting resin, has been employed for a number of years in the manufacture of laminated composite structural shapes for aircraft and aerospace vehicles. Composite tape structural parts are commonly made by superposing many laminations with different tape orientations, each lamination made by laying down strips of composite tape in a side-by-side relation upon a pattern or form. The many laminations are placed in a mold and heated to cure the resin and provide the finished part. Much time, effort, and expense has been devoted to the fabrication of automatic machines for the laying of individual strips of tape. Examples of such tape laying machines are shown in U.S. Pat. Nos. 3,574,040; 3,775,219; 3,810,805; 4,133,711; and 4,292,108.
These machines generally involve an overhead gantry from which is suspended a laterally traveling tape laying head which often carries tape supply and take up reels, guiding mechanisms, cutting mechanisms, a heater, and a tape pressure foot. The tape laying head does all the tape preparation, in addition to pressing the tape upon a work surface. In the head mechanism, tape is withdrawn from a supply reel carried by the mechanism and cut into predetermined strip lengths as it is applied to a work surface. One part of a continuous tape length is often laid down before the trailing end of the tape is cut. The composite tape, having an exceedingly high modulus of elasticity, is stiff, inelastic, and difficult to handle. The tape head laying machine is massive, expensive, slow, and complex, and, in some cases, may comprise a structure in the order of one to two feet in diameter and four to six feet in height. All of this structure must be movably mounted on a gantry and controlled for guiding the tapes in precise, aligned side-by-side paths.
In such prior machines, each cut made in a continuous tape defines both the trailing edge of one tape strip and the immediately adjoining leading edge of the next tape strip. Where the tape is to be cut at an angle, so as to closely follow the angulated boundary of a part that is being formed, the tape pressure foot or roller cannot press to the trailing end of one angularly cut strip without pressing at least part of the immediately adjoining leading edge of the following strip. This is undesirable, because the leading edge of the following strip must be positioned at a different location than the trailing edge of the first strip. Further, for smaller oblique angles of cut, the length of cut increases, and, accordingly, the force required to complete the cut increases, thus further adding to complexity and bulk of the prior art tape laying machine.
An additional problem exists in the laying of tape upon a surface of compound curvature. To properly fit a compound curve, the individual tape fibers must slip longitudinally relative to one another, since those fibers extending over a path of greater curvature must have a greater length than adjoining fibers of the same tape strip that extend over a path of lesser curvature. However, where the tape supply reel is carried by the tape laying machine, and, in particular, where a long strip of tape is not cut until after a portion of the same strip has been laid, no relative slippage of tape fibers is possible.
In prior machines, cutting of the tape, which is generally mounted upon a paper backing ribbon, is achieved either by first removing the paper backing to cut the tape without cutting the ribbon, or by cutting the tape with a carefully controlled cutter that does not sever the backing ribbon. Because in such systems further handling of the tape on the backing ribbon is required, it is necessary either to cut the tape without cutting the ribbon or to replace the tape upon the ribbon after the cutting.
Imperfections in the manufacture of the tape, or damage to the tape during handling, can be discovered in prior art machines only after the tape has been pulled out of the tape laying machine and at least partly laid upon the work surface. Therefore, a damaged tape strip can be replaced only after it has been laid, a situation that increases cost and time of manufacture, particularly where a damaged strip of tape of many feet or many tens of feet in length must be removed after it has been laid.
Accordingly, it is an object of this invention to avoid or minimize abovementioned problems.